Thompson, whose companies received hundreds of millions of dollars in city and federal contracts, admitted to pumping hundreds of thousands of dollars in illegal campaign contributions to various candidates since 2006.

His role in the shadow campaign has been a critical focus of the ongoing criminal probe for three years, and on Monday he admitted to funneling $3.3 million in illegal contributions to 28 candidates, including candidates running for mayor and President of the United States.

Thompson on Monday admitted to meeting with Gray and secretly channeling more than $668,800 to pay for campaign activities for the Gray campaign, including to pay for thousands of T-shirts, placards and bumper stickers.

In court Monday, prosecutors said Thompson asked Gray to call him “Uncle Earl” to hide his identity.

Gray remains uncharged as he battles to stay atop of the polls in the upcoming 2014 mayoral election. Voting starts next week.

The mayor sat down with NBC4’s longtime reporter Tom Sherwood to address the allegations.

The mayor said Thompson asked to be called “Uncle Earl” because Thompson didn’t want the then-incumbent mayor Adrian Fenty to know that the contractor was supporting his opponent.

Gray said he believed that Thompson’s support was all above board at the time.

Thompson has agreed to cooperate with investigators. The plea deal calls for a sentence of six months behind bars.

“Election after election, Jeff Thompson huddled behind closed doors with corrupt candidates, political operatives, and businessmen, devising schemes to funnel millions of dollars of coporate money into local and federal elections,” U.S. Attorney Ronald Machen Jr., said in a statement. “Today’s guilty plea pulls back the curtain on years of widespread corruption.”

With Thompson’s cooperation, Machen said, prosecutors possess “the opportunity to hold many wrongdoers accountable and to usher in a new era of honesty, integrity and transparency in D.C. politics.”

Prosecutors have not said whether they believe Gray knew about the arrangement.

To keep track of the hundreds of thousands of dollars flowing from numerous straw donors into various political campaigns, Thompson and his firm created a special accounting system, prosecutors have said.

Thompson, 58, is the majority owner of the accounting, tax and management firm Thompson, Cobb, Bazilio and Associates, which receives millions of dollars in contracts from D.C. and federal entities.

He is also the owner of D.C. Healthcare Systems, which has a $300 million a year contract with the District to provide managed care to D.C. residents.